這篇文章介紹filter的工作原理。配置方式爲xml。
Filter如何進入執行邏輯的
初始配置：
<filter>
<filter-name>springSecurityFilterChain</filter-name>
<filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
</filter>
<filter-mapping>
<filter-name>springSecurityFilterChain</filter-name>
<url-pattern>/*</url-pattern>
</filter-mapping>DelegatingFilterProxy這個類繼承了GenericFilterBean，GenericFilterBean實現了Filter接口。
這個配置是一切的開始，配置完這個之後，在啓動項目的時候會執行Filterd的初始化方法：
@Override
public final void init(FilterConfig filterConfig) throws ServletException {
Assert.notNull(filterConfig, "FilterConfig must not be null");
if (logger.isDebugEnabled()) {
logger.debug("Initializing filter '" + filterConfig.getFilterName() + "'");
}
this.filterConfig = filterConfig;
// Set bean properties from init parameters.
PropertyValues pvs = new FilterConfigPropertyValues(filterConfig, this.requiredProperties);
if (!pvs.isEmpty()) {
try {
BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
ResourceLoader resourceLoader = new ServletContextResourceLoader(filterConfig.getServletContext());
Environment env = this.environment;
if (env == null) {
env = new StandardServletEnvironment();
}
bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, env));
initBeanWrapper(bw);
bw.setPropertyValues(pvs, true);
}
catch (BeansException ex) {
String msg = "Failed to set bean properties on filter '" +
filterConfig.getFilterName() + "': " + ex.getMessage();
logger.error(msg, ex);
throw new NestedServletException(msg, ex);
}
}
// Let subclasses do whatever initialization they like.
initFilterBean(); // 這個方法
if (logger.isDebugEnabled()) {
logger.debug("Filter '" + filterConfig.getFilterName() + "' configured successfully");
}
}在初始化方法中，會執行初始化Filter的方法initFilterBean。這個方法的實現在DelegatingFilterProxy中：
protected void initFilterBean() throws ServletException {
synchronized (this.delegateMonitor) {
if (this.delegate == null) {
// If no target bean name specified, use filter name.
if (this.targetBeanName == null) {
this.targetBeanName = getFilterName();
}
// Fetch Spring root application context and initialize the delegate early,
// if possible. If the root application context will be started after this
// filter proxy, we'll have to resort to lazy initialization.
WebApplicationContext wac = findWebApplicationContext();
if (wac != null) {
this.delegate = initDelegate(wac); //這個方法
}
}
}
}在這個初始化方法中又調用initDelegate方法進行初始化：
protected Filter initDelegate(WebApplicationContext wac) throws ServletException {
String targetBeanName = getTargetBeanName();
Assert.state(targetBeanName != null, "No target bean name set");
Filter delegate = wac.getBean(targetBeanName, Filter.class);
if (isTargetFilterLifecycle()) {
delegate.init(getFilterConfig());
}
return delegate;
}在這個方法中，先獲取targetBeanName，這個名字是構造方法中賦值的：
public DelegatingFilterProxy(String targetBeanName, @Nullable WebApplicationContext wac) {
Assert.hasText(targetBeanName, "Target Filter bean name must not be null or empty");
this.setTargetBeanName(targetBeanName);
this.webApplicationContext = wac;
if (wac != null) {
this.setEnvironment(wac.getEnvironment());
}
}這個名字就是web.xml中配置的名字springSecurityFilterChain：
<filter>
<filter-name>springSecurityFilterChain</filter-name>
<filter-class>org.springframework.web.filter.DelegatingFilterProxy</filter-class>
</filter>springSecurityFilterChain是固定不能改的，如果改了啓動時就會報錯，這是spring 啓動時內置的一個bean，這個bean實際是FilterChainProxy。
這樣一個Filter就初始化話好了，過濾器chain也初始化好了。
當一個請求進來的時候，會進入FilterChainProxy執行doFilter方法：
public void doFilter(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
boolean clearContext = request.getAttribute(FILTER_APPLIED) == null;
if (clearContext) {
try {
request.setAttribute(FILTER_APPLIED, Boolean.TRUE);
doFilterInternal(request, response, chain);
}
finally {
SecurityContextHolder.clearContext();
request.removeAttribute(FILTER_APPLIED);
}
}
else {
doFilterInternal(request, response, chain);
}
}先獲取所有的Filter，然後執行doFilterInternal方法：
private void doFilterInternal(ServletRequest request, ServletResponse response,
FilterChain chain) throws IOException, ServletException {
FirewalledRequest fwRequest = firewall
.getFirewalledRequest((HttpServletRequest) request);
HttpServletResponse fwResponse = firewall
.getFirewalledResponse((HttpServletResponse) response);
List<Filter> filters = getFilters(fwRequest);
if (filters == null || filters.size() == 0) {
if (logger.isDebugEnabled()) {
logger.debug(UrlUtils.buildRequestUrl(fwRequest)
+ (filters == null ? " has no matching filters"
: " has an empty filter list"));
}
fwRequest.reset();
chain.doFilter(fwRequest, fwResponse);
return;
}
// 最終執行下面的這些代碼
VirtualFilterChain vfc = new VirtualFilterChain(fwRequest, chain, filters);
vfc.doFilter(fwRequest, fwResponse);
}VirtualFilterChain是一個匿名內部類：
private static class VirtualFilterChain implements FilterChain {
private final FilterChain originalChain;
private final List<Filter> additionalFilters;
private final FirewalledRequest firewalledRequest;
private final int size;
private int currentPosition = 0;
private VirtualFilterChain(FirewalledRequest firewalledRequest,
FilterChain chain, List<Filter> additionalFilters) {
this.originalChain = chain;
this.additionalFilters = additionalFilters;
this.size = additionalFilters.size();
this.firewalledRequest = firewalledRequest;
}
@Override
public void doFilter(ServletRequest request, ServletResponse response)
throws IOException, ServletException {
if (currentPosition == size) {
if (logger.isDebugEnabled()) {
logger.debug(UrlUtils.buildRequestUrl(firewalledRequest)
+ " reached end of additional filter chain; proceeding with original chain");
}
// Deactivate path stripping as we exit the security filter chain
this.firewalledRequest.reset();
originalChain.doFilter(request, response);
}
else {
currentPosition++;
Filter nextFilter = additionalFilters.get(currentPosition - 1);
if (logger.isDebugEnabled()) {
logger.debug(UrlUtils.buildRequestUrl(firewalledRequest)
+ " at position " + currentPosition + " of " + size
+ " in additional filter chain; firing Filter: '"
+ nextFilter.getClass().getSimpleName() + "'");
}
nextFilter.doFilter(request, response, this);
}
}
}filter集合執行的邏輯在VirtualFilterChain的doFilter方法中。
filter是如何執行的
上面說了怎麼才能進入filter的執行邏輯，下面說一下filter到底怎麼執行，爲什麼一個
在VirtualFilterChain的doFilter方法可以執行所有的filter。
下面寫一個例子，模擬filter的執行邏輯。
定義FilterChain接口、Filter接口：
public interface Filter {
void doFilter(String username, int age, FilterChain filterChain);
}public interface FilterChain {
void doFilter(String username, int age);
}定義兩個Filter實現：
public class NameFilter implements Filter {
@Override
public void doFilter(String username, int age, FilterChain filterChain) {
username = username + 1;
System.out.println("username: " + username + " age: " + age);
System.out.println("正在執行：NameFilter");
filterChain.doFilter(username, age);
}
}public class AgeFilter implements Filter {
@Override
public void doFilter(String username, int age, FilterChain filterChain) {
age += 10;
System.out.println("username: " + username + " age: " + age);
System.out.println("正在執行：AgeFilter");
filterChain.doFilter(username, age);
}
}定義一個FilterChain實現：
public class FilterChainProxy implements FilterChain {
private int position = 0;
private int size = 0;
private List<Filter> filterList = new ArrayList<>();
public void addFilter(Filter filter) {
filterList.add(filter);
size++;
}
@Override
public void doFilter(String username, int age) {
if (size == position) {
System.out.println("過濾器鏈執行結束");
} else {
Filter filter = filterList.get(position);
position++;
filter.doFilter(username, age, this);
}
}
}測試Filter實現：
public class FilterTest {
public static void main(String[] args) {
FilterChainProxy proxy = new FilterChainProxy();
proxy.addFilter(new NameFilter());
proxy.addFilter(new AgeFilter());
proxy.doFilter("張三", 0);
}
}
=======
username: 張三1 age: 0
正在執行：NameFilter
username: 張三1 age: 10
正在執行：AgeFilter
過濾器鏈執行結束在這個執行邏輯中，最重要的是【this】，this就是初始化的好的FilterChain實例，在這個測試實例中，this就是FilterChainProxy。
執行FilterChainProxy的doFilter方法的時候，傳入了初始參數username和age，進入這個方法後，根據position取出相應的Filter，初次進入position是0，執行Filter的doFilter方法，注意，此時Filter的doFilter方法額外傳入了一個this參數，這個參數就是初始化的好的FilterChain實例，在Filter中的doFilter的方法中最後又會執行FilterChain的doFilter方法，相當於第二次調用FilterChain實例的doFilter方法，此時posotion是1，然後再執行Filter的doFilter方法，直到所有的Filter執行完，整個執行過程結束。
VirtualFilterChain的doFilter方法的執行邏輯和這個測試實例中的執行邏輯基本一致。
這樣就完成了整個過濾器鏈的執行。
總結
以前用Filter的時候就非常疑惑過濾器怎麼執行的，直到今天才算解決了這個疑惑。